U.S. patent number 7,455,787 [Application Number 10/632,747] was granted by the patent office on 2008-11-25 for etching of solar cell materials.
This patent grant is currently assigned to Sunpower Corporation. Invention is credited to Douglas H. Rose, David D. Smith, Pongsthorn Uralwong.
United States Patent |
7,455,787 |
Rose , et al. |
November 25, 2008 |
Etching of solar cell materials
Abstract
A solar cell is fabricated by etching one or more of its layers
without substantially etching another layer of the solar cell. In
one embodiment, a copper layer in the solar cell is etched without
substantially etching a topmost metallic layer comprising tin. For
example, an etchant comprising sulfuric acid and hydrogen peroxide
may be employed to etch the copper layer selective to the tin
layer. A particular example of the aforementioned etchant is a
Co-Bra Etcho.RTM. etchant modified to comprise about 1% by volume
of sulfuric acid, about 4% by volume of phosphoric acid, and about
2% by volume of stabilized hydrogen peroxide. In one embodiment, an
aluminum layer in the solar cell is etched without substantially
etching the tin layer. For example, an etchant comprising potassium
hydroxide may be employed to etch the aluminum layer without
substantially etching the tin layer.
Inventors: |
Rose; Douglas H. (Austin,
TX), Uralwong; Pongsthorn (Campbell, CA), Smith; David
D. (San Jose, CA) |
Assignee: |
Sunpower Corporation (San Jose,
CA)
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Family
ID: |
34104464 |
Appl.
No.: |
10/632,747 |
Filed: |
August 1, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20050022861 A1 |
Feb 3, 2005 |
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Current U.S.
Class: |
216/24; 216/83;
216/95; 216/41; 216/100; 136/256 |
Current CPC
Class: |
H01L
31/18 (20130101); H01L 31/02008 (20130101); H01L
31/022425 (20130101); Y02E 10/50 (20130101) |
Current International
Class: |
C23F
1/00 (20060101) |
Field of
Search: |
;216/24,41,83
;136/256 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
PCT International Search Report for Application No.
PCT/US2004/023198, no date provided. cited by other .
W.P. Mulligan, et al., "A Flat-Plate Concentrator:
Micro-Concentrator Design Overview", 2000, 3 sheets; Proceedings of
the 28.sup.th IEEE PVSC. cited by other .
Keith R. McIntosh, et al., "The Choice of Silicon Wafer for the
Production of Low-Cost Rear-Contact Solar Cells", May 2003, 4
sheets; Sunnyvale, California. cited by other .
P.J. Verlinden, et al., "Will We Have a 20%-Efficient (PTC)
Photovoltaic System?", 2001, 6 sheets; Proceedings of the 17.sup.th
Europe Photovoltaic Solar Energy Conference. cited by other .
William P. Mulligan, et al., "Development of Chip-Size Silicon
Solar Cells", 2000, 6 sheets; Proceedings of the 28.sup.th IEEE
PVSC. cited by other .
Akira Terao, et al., "A Mirror-Less Design for Micro-Concentrator
Modules", 2000, 4 sheets; Proceedings of the 28.sup.th IEEE PVSC.
cited by other .
P.J. Verlinden, et al., "Backside-Contact Silicon Solar Cells with
Improved Efficiency for the '96 World Solar Challenge", 1997, 5
sheets; Proceedings of the 15.sup.th EPSEC. cited by other .
P.J. Verlinden, et al., "One-Year Comparison of a Concentrator
Module with Silicon Point-Contact Solar Cell to a Fixed Flat Plate
Module in Northern California", 2000, 4 sheets; Proceedings of the
16.sup.th EPSEC. cited by other .
Richard M. Swanson, "The Promise of Concentrators", 2000, pp.
93-111, Progress in Photovoltaics: Research and Applications. cited
by other.
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Primary Examiner: Ahmed; Shamim
Attorney, Agent or Firm: Okamoto & Benedicto LLP
Claims
What is claimed is:
1. A method of fabricating a solar cell, the method comprising:
etching a first layer comprising copper without substantially
etching a topmost metallic layer comprising tin on a backside of a
solar cell, the backside of the solar cell being opposite a front
side for collecting solar radiation, the topmost metallic layer
providing a solderable metallic surface for electrically coupling
the solar cell to an external electrical circuit and protecting a
second layer during etching of the first layer, the second layer
being between the topmost metallic layer and the first layer.
2. The method of claim 1 wherein the first layer is etched using an
etchant comprising sulfuric acid and hydrogen peroxide.
3. The method of claim 1 wherein the first layer is etched using an
etchant comprising about 1% by volume of sulfuric acid, about 4% by
volume of phosphoric acid, and about 2% by volume of stabilized
hydrogen peroxide.
4. The method of claim 1 wherein the first layer is etched using an
etchant comprising about 1% by volume of sulfuric acid, about 4% by
volume of phosphoric acid, and about 2% by volume of stabilized
hydrogen peroxide.
5. The method of claim 1 wherein the topmost metallic layer is
etched using an etchant comprising sulfuric acid and hydrogen
peroxide.
6. The method of claim 1 further comprising: etching a second layer
comprising titanium-tungsten using an etchant comprising hydrogen
peroxide.
7. The method of claim 6 further comprising: etching a third layer
comprising aluminum using an etchant comprising potassium
hydroxide.
8. The method of claim 1 further comprising: etching a second layer
comprising aluminum using an etchant comprising potassium
hydroxide.
9. The method of claim 8 wherein the etchant comprises about 1% by
volume of potassium hydroxide in water.
10. A method of etching a layer of material in a solar cell, the
method comprising: etching a first copper layer selective to a tin
layer on a backside of a solar cell using an etchant comprising
sulfuric acid and hydrogen peroxide, the backside of the solar cell
being opposite a front side for collecting solar radiation, the tin
layer being a topmost metallic layer and configured to provide a
solderable metallic surface and to protect a second layer between
the tin layer and the first copper layer during etching of the
first copper layer.
11. The method of claim 10 wherein the etchant comprises about 1%
by volume of sulfuric acid, about 4% by volume of phosphoric acid,
and about 2% by volume of stabilized hydrogen peroxide.
12. A method of etching a layer of material in a solar cell, the
method comprising: etching a first metal layer comprising copper
without substantially etching a tin layer on a backside of a solar
cell, the backside being opposite a front side for collecting solar
radiation, the tin layer being configured to provide a solderable
metallic surface and to protect a second metal layer during etching
of the first metal layer, the second layer being between the tin
layer and the first metal layer.
13. The method of claim 12 wherein the first metal layer is etched
using an etchant comprising about 1% by volume of sulfuric acid,
about 4% by volume of phosphoric acid, and about 2% by volume of
stabilized hydrogen peroxide.
14. The method of claim 12 wherein the first metal layer is etched
using an etchant comprising hydrogen peroxide and sulfuric acid.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to solar cells, and more
particularly but not exclusively to solar cell fabrication
processes and structures.
2. Description of the Background Art Solar cells are well known
devices for converting solar radiation to electrical energy. They
may be fabricated on a semiconductor wafer using semiconductor
processing technology. Generally speaking, a solar cell may be
fabricated by forming p-doped and n-doped regions in a silicon
substrate. Solar radiation impinging on the solar cell creates
electrons and holes that migrate to the p-doped and n-doped
regions, thereby creating voltage differentials between the doped
regions. The side of the solar cell where connections to an
external electrical circuit are made includes a topmost metallic
surface that is electrically coupled to the doped regions. There
may be several layers of materials between the metallic surface and
the doped regions. These materials may be patterned and etched to
form internal structures. It is important to etch these materials
in a way that would not compromise the operability and performance
of the solar cell.
SUMMARY
A solar cell is fabricated by etching one or more of its layers
without substantially etching another layer of the solar cell. In
one embodiment, a copper layer in the solar cell is etched without
substantially etching a topmost metallic layer comprising tin. For
example, an etchant comprising sulfuric acid and hydrogen peroxide
may be employed to etch the copper layer selective to the tin
layer. A particular example of the aforementioned etchant is a
Co-Bra Etch.RTM. etchant modified to comprise about 1% by volume of
sulfuric acid, about 4% by volume of phosphoric acid, and about 2%
by volume of stabilized hydrogen peroxide. In one embodiment, an
aluminum layer in the solar cell is etched without substantially
etching the tin layer. For example, an etchant comprising potassium
hydroxide may be employed to etch the aluminum layer without
substantially etching the tin layer.
These and other features of the present invention will be readily
apparent to persons of ordinary skill in the art upon reading the
entirety of this disclosure, which includes the accompanying
drawings and claims.
DESCRIPTION OF THE DRAWINGS
FIGS. 1-4 show sectional views schematically illustrating the
fabrication of a solar cell in accordance with an embodiment of the
present invention.
FIG. 5 shows a flow diagram of a method of etching one or more
materials in a solar cell in accordance with an embodiment of the
present invention.
The use of the same reference label indifferent drawings indicates
the same or like components.
DETAILED DESCRIPTION
In the present disclosure, numerous specific details are provided
such as examples of process parameters, materials, process steps,
and structures to provide a thorough understanding of embodiments
of the invention. Persons of ordinary skill in the art will
recognize, however, that the invention can be practiced without one
or more of the specific details. In other instances, well-known
details are not shown or described to avoid obscuring aspects of
the invention.
FIGS. 1-4 show sectional views schematically illustrating the
fabrication of a solar cell in accordance with an embodiment of the
present invention. FIGS. 1-4, which are not drawn to scale, show
the solar cell in the middle of the fabrication process. Masking
steps are not shown or described in the interest of clarity.
In FIG. 1, the solar cell is shown as having a tin (Sn) layer 112,
a copper (Cu) layer 110, a copper layer 108, a titanium-tungsten
(TiW) layer 106, an aluminum (Al) layer 104, a silicon dioxide
(SiO.sub.2) layer 102, and a silicon (Si) substrate 100. Tin layer
112 is on the backside of the solar cell, while silicon substrate
100 is towards the front or sun side. The solar cell being
fabricated is a so-called "backside-contact solar cell" in that all
electrical connections to the doped regions (not shown) in silicon
substrate 100 are made from the backside of the solar cell by way
of tin layer 112. However, it is to be noted that the present
invention is not limited to backside-contact solar cells. The
teachings of the present disclosure may be employed in the
fabrication of solar cells in general.
In the example of FIG. 1, tin layer 112 is the topmost metallic
layer and provides a solderable metallic surface on which
electrical connections may be made. For example, interconnect leads
coupled to an external electrical circuit or other solar cells may
be soldered on tin layer 112. Tin layer 112 also protects
underlying layers of materials. For example, tin layer 112 helps
prevent copper layer 110 from corroding. In one embodiment, tin
layer 112 is electroplated to a thickness of about 5 microns on
copper layer 110.
Copper layer 110, copper layer 108, titanium-tungsten layer 106,
and aluminum layer 104 form a Cu/TiW/Al metal stack that provides
electrical connectivity to doped regions in silicon substrate 100.
In one embodiment, copper layer 110 is electroplated to a thickness
of about 20 microns on copper layer 108. Masks (not shown) may be
formed between individual structures of copper layer 110 in gaps
113 before the electroplating process. The masks are removed after
the electroplating process to obtain the structure shown in FIG.
1.
Copper layer 108 serves as a seed layer for the electroplating of
copper layer 110. Copper layer 108 may be formed to a thickness of
about 1600 Angstroms by sputtering. Titanium-tungsten layer 106 and
aluminum layer 104 may also be formed by sputtering. In one
embodiment, titanium-tungsten layer 106 and aluminum layer 104 are
each formed to a thickness of about 1000 Angstroms. Aluminum layer
104 may comprise aluminum with 1% silicon alloy.
Silicon dioxide layer 102 serves as a dielectric layer providing
electrical isolation between the overlying Cu/TiW/Al metal stack
and silicon substrate 100. Vias are formed through silicon dioxide
layer 102 in sections where the Cu/TiW/Al metal stack makes contact
with the doped regions in silicon substrate 100. In one embodiment,
silicon dioxide layer 102 is formed to a thickness of about 950
Angstroms.
There may be steps in the fabrication of a solar cell where an etch
is performed through a stack of materials comprising copper,
titanium-tungsten, and aluminum. To prevent damaging the solar
cell, each layer in the material stack may need to be etched
without attacking (i.e., excessively etching) other layers of the
solar cell. In the example of FIG. 1, copper layer 108,
titanium-tungsten layer 106, and aluminum layer 104 need to be
etched without attacking tin layer 112. Excessive etching of tin
layer 112 may compromise its ability to protect the underlying
metal stack.
One way of etching through copper layer 108, titanium-tungsten
layer 106, and aluminum layer 104 is to use a so-called "PAWN"
(phosphoric, acetic, water, nitric) solution to etch copper layer
108 and aluminum layer 104. For example, the sample of FIG. 1 may
be wet etched in a PAWN bath to etch copper layer 108, in a
hydrogen peroxide bath to etch titanium-tungsten layer 106, and in
a PAWN bath to etch aluminum layer 104. PAWN is commonly used to
etch aluminum in the electronics industry, which does not require
selectivity to copper, tin, and other materials employed in solar
cells. The inventors found that PAWN has a tendency to attack tin
layer 112 during the etching of copper layer 108 and aluminum layer
104. The present disclosure provides improved techniques for
etching a copper layer, a titanium-tungsten layer, and/or aluminum
layer without substantially etching a tin layer in a solar cell or
similar device. The techniques may be employed to etch several
layers of a material stack or a single layer.
Continuing in FIG. 2, copper layer 108 is etched selective to tin
layer 112 and titanium-tungsten layer 106. Copper layer 108 may be
wet etched in a solution comprising sulfuric acid and hydrogen
peroxide. For example, copper layer 108 may be wet etched using a
Co-Bra Etch.RTM. etchant modified to comprise about 1% by volume of
sulfuric acid, about 4% by volume of phosphoric acid, and about 2%
by volume of stabilized hydrogen peroxide. A Perma-Etch.RTM.
etchant may also be employed to etch copper layer 108. Co-Bra
Etch.RTM. and Perma-Etch.RTM. etchants are both commercially
available from Electrochemicals, Inc. of Maple Plain, Minn. In one
experiment, a sample similar to that shown in FIG. 1 was dipped in
a bath of the aforementioned modified Co-Bra Etch.RTM. etchant for
about 1 minute at 40.degree. C. About 1600 Angstroms of copper
layer 108 were removed without excessively etching tin layer 112 or
titanium-tungsten layer 106.
In FIG. 3, titanium-tungsten layer 106 is etched selective to tin
layer 112, copper layers 110 and 108, and aluminum layer 104.
Titanium-tungsten layer 106 may be wet etched in a bath of hydrogen
peroxide, for example. The hydrogen peroxide may be 30% by weight
(balance is water).
In FIG. 4, aluminum layer 104 is etched selective to tin layer 112,
copper layers 110 and 108, titanium-tungsten layer 106, and silicon
dioxide layer 102. Aluminum layer 104 may be wet etched in a bath
of potassium hydroxide. In one embodiment, aluminum layer 104 is
wet etched using an etchant comprising about 1% by volume of
potassium hydroxide in water. Other concentrations of potassium
hydroxide may also be employed depending on the application. In one
experiment, a sample similar to that shown in FIG. 3 was dipped in
a bath comprising about 1% by volume of potassium hydroxide in
water for about 1.5 minutes at 40.degree. C. About 1000 Angstroms
of aluminum layer 104 were removed without excessively etching tin
layer 112, copper layers 110 and 108, titanium-tungsten layer 106,
and silicon dioxide layer 102.
The teachings of the present disclosure may be generally employed
to etch one or more layers of materials in a solar cell being
fabricated. For example, the etching techniques disclosed herein
may be employed in the fabrication of solar cells disclosed in the
following commonly-assigned disclosures, which are incorporated
herein by reference in their entirety: U.S. application Ser. No.
10/412,638, entitled "Improved Solar Cell and Method of
Manufacture," filed on Apr. 10, 2003 by William P. Mulligan,
Michael J. Cudzinovic, Thomas Pass, David Smith, Neil Kaminar,
Keith McIntosh, and Richard M. Swanson; and U.S. application Ser.
No. 10/412,711, entitled "Metal Contact Structure For Solar Cell
And Method Of Manufacture," filed on Apr. 10, 2003 by William P.
Mulligan, Michael J. Cudzinovic, Thomas Pass, David Smith, and
Richard M. Swanson. It is to be noted, however, that the
aforementioned disclosures are referenced herein only as
examples.
The etch chemistries provided herein not only allow selectivity to
materials found in solar cells, but also have relatively high etch
capacity, are cost-effective, and are easily replenished and
controlled. Embodiments, of the present invention may thus be
advantageously employed to etch a single layer of material or a
stack of materials in solar cell fabrication processes in
general.
Referring now to FIG. 5, there is shown a flow diagram of a method
500 of etching one or more materials in a solar cell in accordance
with an embodiment of the present invention. Among its other uses,
method 500 may be employed to etch through a material stack
comprising copper, titanium-tungsten, and aluminum in a solar cell.
Method 500 will be described using FIGS. 1-4 as an example.
In step 502 and with reference to FIG. 1, a plating mask, if any,
that may have been employed in the electroplating of copper layer
110 is removed. Step 502 may be performed by placing the sample of
FIG. 1 in about 2% to 3% potassium hydroxide bath at 40.degree. C.
for about 5 minutes.
In step 504, the sample of FIG. 1 is rinsed. For example, the
sample of FIG. 1 may be placed in a bath of deionized water,
spray-rinsed with deionized water, and then dumped-rinsed in
deionized water.
In step 506, copper layer 108 is etched as shown in FIG. 2. Step
506 may be performed by placing the sample of FIG. 1 in a bath of a
Co-Bra Etch.RTM. etchant modified to comprise about 1% by volume of
sulfuric acid, about 4% by volume of phosphoric acid, and about 2%
by volume of stabilized hydrogen peroxide at 43.degree. C. with
robotic agitation for about 2 minutes and 10 seconds.
In step 508, the sample of FIG. 2 is rinsed. Step 508 may be
performed by dump-rinsing the sample of FIG. 2 in deionized
water.
In step 510, titanium-tungsten layer 106 is etched as shown in FIG.
3. Step 510 may be performed by placing the sample of FIG. 3 in a
bath of about 30% by weight hydrogen peroxide at 40.degree.C. with
robotic agitation for about 2 minutes and 15 seconds.
In step 512, the sample of FIG. 3 is rinsed. Step 512 may be
performed by dump-rinsing the sample of FIG. 3 in deionized
water.
In step 514, aluminum layer 104 is etched as shown in FIG. 4. Step
514 may be performed by placing the sample of FIG. 3 in a bath of
about 1% by volume potassium hydroxide at 40.degree. C. for about 2
minutes and 15 seconds.
In step 516, the sample of FIG. 4 is rinsed. Step 516 may be
performed by dump-rinsing the sample of FIG. 4 in deionized
water.
In step 518, the sample of FIG. 4 is dried. Step 518 may be
performed by spin-rinse drying. Air-knife and meniscus drying
techniques may also be employed to dry the sample of FIG. 4.
While specific embodiments of the present invention have been
provided, it is to be understood that these embodiments are for
illustration purposes and not limiting. For example, the above
described etchants may also be applied using in-line drag-through
and in-line spray systems. Many additional embodiments will be
apparent to persons of ordinary skill in the art reading this
disclosure.
* * * * *